50 Most Important MCQ Question of Synchronous Motor with Explanation | Objective Type Question of Synchronous Motor

Ques 1. Synchronous motor for power factor correction operates at

  1. No load with over-excited fields
  2. No load with under-excited fields
  3. Normal load with minimum excitation
  4. Normal load with zero excitation

Answer 1. No load with over-excited fields

Explanation:

  •  A “synchronous condenser” (sometimes called a synchronous capacitor or synchronous compensator) is a device, identical to a synchronous motor, whose shaft is not connected to any loaf but spins freely.
  • It does not to convert electric power to mechanical power or vice versa, but its function is to adjust conditions on the electric power transmission grid.
  • Its field is controlled by a voltage regulator to either generate or absorb reactive power as needed to adjust the grid’s voltage or to improve power factor. The condenser’s installation and operation are identical to large electric motors and generators.
  • Increasing the device’s field winding excitation results in its furnishing reactive power (VARs) to the system and decreasing the field winding excitation causes absorption of reactive power from the system (VARs).
  • Hence, it acts as a capacitor in over excited mode and an as an inductor in under-excited mode.

 

Ques 2. What happens if field winding of the synchronous motor is short circuited?

  1. First, starts as induction motor then run as synchronous motor
  2. Not start
  3. Motor will burn out
  4. Run as induction motor

Answer 4. Run as induction motor

Explanation:

  • In the case of an induction motor, the field winding is permanently short circuited by end rings.
  • In a Synchronous motor, the rotor winding is supplied with DC excitation. Now, if we eliminate the DC excitation, there are two ends of the wire available.
  • Let short circuit the two ends of the wire (or connect the two ends of variable resistors).
  • Now the rotor winding of the synchronous motor is analogous to that of an induction motor.
  • Therefore if we short circuit the field winding of a synchronous motor then it will behave as an induction motor.

 

Ques 3. When the field winding of an unloaded salient pole synchronous motor is open circuited the motor will

  1. Burn
  2. Not start
  3. Run as induction Motor
  4. Run as variable reluctance motor

Answer 4. Run as variable reluctance motor

Explanation:

  • The rotor will remain in synchronism for a short time ie.. until the residual flux of the rotor decreases below the certain value.
  • Variable reluctance motor behaves as similar to silent pole synchronous motor unexcited.
  • And after the loss of residual flux, it will run as a reluctance motor due to the generation of reluctance torque.

 

Ques 4. The speed regulation of a synchronous motor is

  1. 100%
  2. 50%
  3. 25%
  4. 0%

Answer 4. 0%

Explanation:

  • The speed regulation of a motor expresses how much its speed varies between no-load and full load.
  • Synchronous motor supply to load at a constant speed or rate.
  • The steady state speed of the motor is constant from no load to the maximum torque that the motor can supply (called as pull out torque).

where nnl = no load speed
nfl = full load speed

  • Since the speed of the motor is constant is speed regulation is zero.

 

Ques 5. The negative phase sequence in a three phase synchronous motor exists when the motor is

  1. Supplied with an unbalanced voltage
  2. Supplied with an unbalanced load
  3. Unbalanced system fault
  4. All of the above

Answer 4. All of the above

Explanation:

Negative phase sequence current in Synchronous motor

  • Negative sequence currents are produced because of the unbalanced currents in the power system.
  • The flow of negative sequence currents in electrical machines (generators and motors) are undesirable as these currents generate high and possibly dangerous temperatures in very short time.
  • Phase current and voltage in the three phase system can be represented in the form of three single phase components.
  • Positive sequence components, Negative sequence components, and Zero sequence components.
  • Positive sequence currents exist during the balanced load condition.

Causes and effects of Negative Sequence Components :

The main cause of negative phase sequence components are:-

  • Unbalanced loads in the system
  • Unbalanced system faults (line to ground faults, two phase faults, three line to ground faults, double line to ground faults)
  • Open phases (open circuit faults).

Effects of Negative Sequence Components

  • When the load on the generator becomes unbalanced, negative phase sequence currents flow.
  • The negative sequence components produce a rotating magnetic field which rotates at a synchronous speed in a direction opposite to the direction of rotor field. Hence effectively the relative 
  • Hence effectively the relative speed between the two is double the synchronous speed. Thus double frequency currents are induced in the rotor.
  • This double induced high-frequency currents will rise the rotor temperature very high and damages the machine if operates continuously.

 

Ques 6. In 3 -Φ synchronous motor if one of the phases is short circuited the motor will

  1. Run as before
  2. Overheated and eventually burn
  3. Not start
  4. Burn
Answer 2. Overheated

Explanation:

  • When a 3 phase motor looses one of the phases then ‘single phasing’ occurs.
  • A case when any of the three phases fails, in order to compensate the motor, starts drawing more amount of current heating the motor even in some cases could burn the motor.

    More clearly it depends upon the load:

    1. Less than 1/3rd of rated load, it will continue to operate without any harm.
    2. Higher than 1/3rd of rated load, the motor will continue to operate, but will draw current more than its rated value. The motor temperature will start increasing.
    3. Higher load (near to the rated load), the motor speed will drop gradually to zero.

    Note:-  In this case, thermal overload relay may stop the motor current or fuse may blow. If thermal overload relay/fuses are not provided, the motor may get but burnt.

 

Ques 7. For a synchronous motor,  the breakdown torque will be proportional to

  1. Applied voltage V
  2. V2
  3. 1/V
  4. 1/V2

Answer 1. Applied voltage V

Explanation:

  • Power in synchronous motor is given as
    Power P = 2NT/60 V²
  • From the above equation, it is clear that  torque T ∝ P
  • The pull-out torque with excitation supplied with a constant voltage source which varies directly with motor terminal voltage.
  • Therefore as power varies as applied voltage V, the torque of a synchronous motor is also varies as applied voltage V.

 

Ques 8. In a synchronous motor, during hunting when the rotor speed exceeds the synchronous speed then damper bar develop

  1. Induction generator torque
  2. Harmonic
  3. DC motor torque
  4. Synchronous motor torque

Answer 1. Induction generator torque Explanation:

Explanation:

Hunting in a synchronous motor.

  • A synchronous motor is very sensitive to fluctuation in the load.
  • When the synchronous motor is under no load the stator and rotor poles axes almost coincide with each other.
  • When the motor is loaded the rotor poles axis falls back with respect to the stator.
  • On the other hand, if the rotor poles are suddenly decreased then the rotor poles pull upward or get advanced.
  • Due to the inertia of rotor, it can’t achieve its final position instantaneously.
  • Therefore rotor will oscillate to its final new stable position. This oscillation of rotor is known as hunting.

Use of Damper winding to prevent hunting

  • Hunting can be reduced by using damper winding.
  • When hunting occur, the difference in the speed of stator and rotor poles develops an induces emf in the damper winding, which acts in such a way to suppress the rotor oscillation.
  • The induced emf generated develop induction torque in the synchronous motor.

 

Ques 9. In a synchronous motor, at no load condition, and with normal excitation the armature current drawn by a synchronous motor is

  1. Zero
  2. Lagging applied voltage by 90°
  3. Leading Applied voltage by 90°
  4. In phase with applied voltage

Answer 2. Lagging applied voltage by 90°

Explanation:

  • At no load condition in the practical motor, the armature current at unity power factor(normal excitation) is not zero because some value of armature current is required to produce the torque for countering rotational losses.
  • In case of 100% excitation i.e when E= V the armature current I lag behind V by the small angle φ 

 

Ques 10.  In a synchronous motor, “hunting” may be due to variation in any of the following EXCEPT

  1. Winding friction
  2. Frequency
  3. Load
  4. Supply voltage

Answer 1. Winding friction

Explanation:

Causes of Hunting in Synchronous Machines

  • A sudden change in load.
  • The sudden change in field current.
  • A load containing harmonic torque.
  • Fault in the supply system.
  • Note: Mechanical losses such as friction loss does not cause hunting.